Generally, wireless power transmission technology based on near-field magnetic coupling is technology for wirelessly transmitting power from a power source having a constant frequency to an electronic device. When electric power is applied to a transmitting coil in the source, a non-radiative time-varying magnetic field is formed in certain space around the transmitting coil. Then, when a receiving coil is located within the formed magnetic field, voltage and current are induced to the receiving coil by the time-varying magnetic field, whereby power is wirelessly transmitted.
As an example of wireless power transmission technology, the battery of an electronic device such as a smart phone, a tablet PC, and the like, may be charged by simply placing the devices on a wireless charging pad that generates a time-varying magnetic field at high-frequency. Such wireless power transmission technology may provide high portability, convenience, and stability, compared with a conventional wired charging environment, which uses charging adapters. Besides the wireless charging of the electronic devices, the wireless power transmission technology is expected to substitute for existing wired power transmission in various fields including electric vehicles, a various wearable devices such as Bluetooth earphones and 3D smart glasses, home appliances, underground facilities, buildings, portable medical devices, robots, leisure devices, and the like.
Generally, a wireless power transceiver system using a non-radiative time-varying magnetic field includes a wireless power transmitting device, which has transmitting coils and supplies power using a wireless power transmission method, and a wireless power receiving device, which has receiving coils and charges battery cells using the power wirelessly supplied from the wireless power transmitting devices, or supplies power to various electric devices in real time.
Meanwhile, in such a wireless power transceiver system, the strength of magnetic coupling between transmitting coils and receiving coils may change according to various environmental variables such as the structure of the transmitting coils and the receiving coils concerning the transmitting coils, the geometric arrangement and positions of the transmitting coils and the receiving coils, and the like. When the strength of the magnetic coupling between the transmitting coils and the receiving coils is changed by the environmental variables, optimum conditions for transmitting and receiving power in the wireless power transceiver system may be changed. For example, depending on the position and arrangement of the receiving coil for the transmitting coil, a dead zone, in which the mutual inductance between the two coils becomes zero, may occur, and because induced current cannot be generated in the area in which the mutual inductance between the transmitting coil and the receiving coil is zero, the wireless power transmission may not be performed. Therefore, it is very important to minimize the dead zone in the wireless power transmission process.
Meanwhile, 3-dimensional wireless power transmission technology is technology capable of stable power transmission regardless of the position and arrangement of a receiving coil by reducing an area corresponding to a dead zone by allowing wireless power transmission although the receiver is located arbitrarily in geometry in 3-dimensional space having x, y, and z axes. Such 3-dimensional wireless power transmission technology is being researched mainly for power transmission to human implantable devices such as endoscopic capsules, pace makers, and the like; smart phones using secondary batteries; wireless headsets and wearable data communication equipment; and wearable medical health care devices.
FIG. 1 is a view illustrating an example of a three-axis receiving coil according to a conventional art. The example illustrated in FIG. 1 has been disclosed in “Wireless powering for a self-propelled and steerable endoscopic capsule for stomach inspection (Biosensors and Bioelectronics, vol. 25, pp. 845-851, 2009)” by R. Carta, G. Tortora, J. Thone, B. Lenaerts, P. Valdastri, A. Menciassi, P. Dario, and R. Puers.
The three-axis receiving coil 101 illustrated in FIG. 1 has a disadvantage in that a receiving circuit is complicated because rectifier circuits are included for each receiving coil when a wireless power transmission system is implemented.
FIG. 2 is a view illustrating an example of a transmitting coil in the form of an array according to a conventional art. The example illustrated in FIG. 2 has been disclosed in “A novel mat-based system for position-varying wireless power transfer to biomedical implants (IEEE Transactions on Magnetics, vol. 49, no. 8, pp. 4774-4779, August 2013)” by Q. Xu, H. Wang, Z. Gao, Z.-H. Mao, J. He, and M. Sun.
The array type transmitting coil 201 illustrated in FIG. 2 is capable of power transmission on a power transmission board, but it is difficult to be used when a receiver is slanted according to the transmitting coil.
Recently, a high efficient system using high-frequency AC signals of frequencies greater than several MHz is proposed to solve these problems with regard to wireless power transmission technology based on near-field magnetic coupling. The high efficient system may efficiently transmit power over several meters by using a frequency range of higher than several MHz and by applying self-resonant coils of a high Q-factor by decreasing the resistance loss of a coil.
Meanwhile, in order to implement a high-efficiency wireless power transmission system using a frequency range of several MHz, it is important to design low-loss coils in terms of transmission efficiency. However, during wireless power transmission in a frequency range higher than several MHz, a skin effect, in which current is concentrated in the outer layer of a conducting wire because of eddy currents, is caused. Also, when the spacing between wires is decreased and the number of turns of the wire is increased to enhance the strength of a magnetic field, a proximity effect caused by magnetic field that is generated by neighboring wires, may cause the distribution of current to be constrained to a certain area of the wire.